The invention relates to a computed tomography method in which an examination zone is irradiated by means of a conical radiation beam and in which the radiation source which generates the radiation beam rotates along a circular trajectory around an axis of rotation relative to the examination zone. The invention also relates to a computed tomography apparatus for carrying out the method as well as a computer program for controlling the computed tomography apparatus.
A method of this kind and a corresponding computed tomography apparatus are known from U.S. Pat. No. 6,285,733. The measuring values which are acquired therein by a two-dimensional detector unit are dependent on the intensity in the radiation beam to the other side of the examination zone and are first subjected to a rebinning operation. This operation yields groups of measuring values which are associated with fan beams which are situated in equidistant fan beam planes which extend parallel to one another and to the axis of rotation. After the rebinning operation, the fan beans are composed of rays which traverse a plane, containing the axis of rotation and extending perpendicularly to the fan beam planes of the relevant group, in puncture points which are situated on equidistant connecting lines which extend perpendicularly to the axis of rotation and parallel to one another.
The measuring data produced by the rebinning operation are subsequently subjected to one-dimensional high-pass filtering as well as to backprojection in order to form at least one CT image. The reconstruction of a voxel in the examination zone then takes into account from each group the measuring data of rays having traversed the relevant voxel from different directions. If the voxel is not situated in the central plane defined by the circular trajectory, the rays traverse the voxel at an angle relative to the central plane and each ray traverses the plane associated with its group at an angle other than 90°. The reconstruction taking into account this cone beam geometry is comparatively complex and hence requires a comparatively large amount of calculation time.
For various applications, however, shorter calculation times are desired. This is the case, for example, in CT-guided biopsy (CT=computed tomography), where a biopsy needle is introduced into an object to be examined and the advancing of the biopsy needle is continuously checked on the basis of a series of three-dimensional CT images. In such so-called CT fluoroscopy only very little time is available for the reconstruction of a CT image, notably when use is made of a so-called “sliding-window” technique in which a CT image is updated (while taking into account newly acquired CT data and CT data already used for the reconstruction of the previous CT image) within a period of time which is significantly shorter than the period of time required for the acquisition of the measuring data of a complete CT image.